Substrate polishing metrology using interference signals

ABSTRACT

A method of polishing a substrate includes holding the substrate on a polishing pad with a polishing head, wherein the polishing pad is supported by a platen, creating relative motion between the substrate and the polishing pad to polish a side of the substrate, generating a light beam and directing the light beam towards the substrate to cause the light beam to impinge on the side of the substrate being polished. Light reflected from the substrate is at a detector to generate an interference signal. A measure of uniformity is computed from the interference signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation and claims the benefit of priorityunder 35 U.S.C. Section 120 of pending U.S. patent application Ser. No.11/532,498, filed Sep. 15, 2006, which is a continuation of U.S. patentapplication Ser. No. 11/225,838, filed Sep. 12, 2005, now U.S. Pat. No.7,118,450, which is a continuation of U.S. patent application Ser. No.10/405,421, filed Apr. 1, 2003, now U.S. Pat. No. 7,011,565, which is acontinuation of U.S. patent application Ser. No. 09/863,118, filed May22, 2001, now U.S. Pat. No. 6,910,944, which is a continuation of U.S.patent application Ser. No. 09/519,156, filed Mar. 6, 2000, now U.S.Pat. No. 6,280,290, which is a continuation of U.S. patent applicationSer. No. 09/258,504, filed Feb. 26, 1999, now U.S. Pat. No. 6,045,439,which is a continuation of U.S. patent application Ser. No. 08/689,930,filed Aug. 16, 1996, now U.S. Pat. No. 5,893,796, which is acontinuation-in-part of U.S. patent application Ser. No. 08/605,769,filed Feb. 22, 1996, now U.S. Pat. No. 5,964,643, which is acontinuation-in-part of U.S. patent application Ser. No. 08/413,982,filed Mar. 28, 1995, abandoned. The disclosure of each prior applicationis considered part of and is incorporated by reference in the disclosureof this application.

BACKGROUND

This invention relates generally to semiconductor manufacture, and moreparticularly to a method for using interference signals obtained duringchemical mechanical polishing (CMP).

In the process of fabricating modern semiconductor integrated circuits(ICs), it is necessary to form various material layers and structuresover previously formed layers and structures. However, the priorformations often leave the top surface topography of an in-process waferhighly irregular, with bumps, areas of unequal elevation, troughs,trenches, and/or other surface irregularities. These irregularitiescause problems when forming the next layer. For example, when printing aphotolithographic pattern having small geometries over previously formedlayers, a very shallow depth of focus is required. Accordingly, itbecomes essential to have a flat and planar surface, otherwise, someparts of the pattern will be in focus and other parts will not. In fact,surface variations on the order of less than 1000 Å over a 25×25 mm diewould be preferable. In addition, if the irregularities are not leveledat each major processing step, the surface topography of the wafer canbecome even more irregular, causing further problems as the layers stackup during further processing. Depending on the die type and the size ofthe geometries involved, the surface irregularities can lead to pooryield and device performance. Consequently, it is desirable to effectsome type of planarization, or leveling, of the IC structures. In fact,most high density IC fabrication techniques make use of some method toform a planarized wafer surface at critical points in the manufacturingprocess.

One method for achieving semiconductor wafer planarization or topographyremoval is the chemical mechanical polishing (CMP) process. In general,the chemical mechanical polishing (CMP) process involves holding and/orrotating the wafer against a rotating polishing platen under acontrolled pressure. As shown in FIG. 1, a typical CMP apparatus 10includes a polishing head 12 for holding the semiconductor wafer 14against the polishing platen 16. The polishing platen 16 is covered witha pad 18. This pad 18 typically has a backing layer 20 which interfaceswith the surface of the platen and a covering layer 22 which is used inconjunction with a chemical polishing slurry to polish the wafer 14.However, some pads have only a covering layer and no backing layer. Thecovering layer 22 is usually either an open cell foamed polyurethane(e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface(e.g. Rodel EX2000). The pad material is wetted with the chemicalpolishing slurry containing both an abrasive and chemicals. One typicalchemical slurry includes KOH (Potassium Hydroxide) and fumed-silicaparticles. The platen is usually rotated about its central axis 24. Inaddition, the polishing head is usually rotated about its central axis26, and translated across the surface of the platen 16 via a translationarm 28. Although just one polishing head is shown in FIG. 1, CMP devicestypically have more than one of these heads spaced circumferentiallyaround the polishing platen.

A particular problem encountered during a CMP process is in thedetermination that a part has been planarized to a desired flatness orrelative thickness. In general, there is a need to detect when thedesired surface characteristics or planar condition has been reached.This has been accomplished in a variety of ways. Early on, it was notpossible to monitor the characteristics of the wafer during the CMPprocess. Typically, the wafer was removed from the CMP apparatus andexamined elsewhere. If the wafer did not meet the desiredspecifications, it had to be reloaded into the CMP apparatus andreprocessed. This was a time consuming and labor-intensive procedure.Alternately, the examination might have revealed that an excess amountof material had been removed, rendering the part unusable. There was,therefore, a need in the art for a device which could detect when thedesired surface characteristics or thickness had been achieved, in-situ,during the CMP process.

Several devices and methods have been developed for the in-situdetection of endpoints during the CMP process. For instance, devices andmethods that are associated with the use of ultrasonic sound waves, andwith the detection of changes in mechanical resistance, electricalimpedance, or wafer surface temperature, have been employed. Thesedevices and methods rely on determining the thickness of the wafer or alayer thereof, and establishing a process endpoint, by monitoring thechange in thickness. In the case where the surface layer of the wafer isbeing thinned, the change in thickness is used to determine when thesurface layer has the desired depth. And, in the case of planarizing apatterned wafer with an irregular surface, the endpoint is determined bymonitoring the change in thickness and knowing the approximate depth ofthe surface irregularities. When the change in thickness equals thedepth of the irregularities, the CMP process is terminated. Althoughthese devices and methods work reasonably well for the applications forwhich they were intended, there is still a need for systems whichprovide a more accurate determination of the endpoint.

SUMMARY OF THE INVENTION

In general, in one aspect, a method of polishing a substrate isdescribed. The substrate is held on a polishing pad with a polishinghead, wherein the polishing pad is supported by a platen. A relativemotion is created between the substrate and the polishing pad to polisha side of the substrate. A light beam is generated. The light beam isdirected towards the substrate to cause the light beam to impinge on theside of the substrate being polished. Light reflected from the substrateis received at a detector to generate an interference signal. A measureof uniformity is computed from the interference signal.

In general, in another aspect, a method of polishing a substrate isdescribed. The substrate is held on a polishing pad with a polishinghead, wherein the polishing pad is supported by a platen. Relativemotion is created between the substrate and the polishing pad to polisha side of the substrate. A light beam is generated. The light beam isdirected towards the substrate to cause the light beam to impinge on theside of the substrate being polished. Light reflected from the substrateis received at a detector to generate an interference signal. Acharacterizing waveform for an operator to see, the characterizingwaveform presenting data for a period of time that extends over multiplecycles of the interference signal.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized by means of theinstrumentalities and combinations particularly pointed out in theclaims.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated and constitute a partof the specification, schematically illustrate an embodiment of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the principles of theinvention.

FIG. 1 is a side view of a chemical mechanical polishing (CMP) apparatustypical of the prior art.

FIG. 2 is a side view of a chemical mechanical polishing apparatus withendpoint detection constructed in accordance with the present invention.

FIGS. 3A-D are simplified cross-sectional views of respectiveembodiments of the window portion of the apparatus of FIG. 2.

FIG. 3E is a simplified top view of the transparent plug used in thewindow portion of FIG. 3D.

FIG. 3F is a simplified cross-sectional view illustrating the assemblyof the window portion of FIG. 3D.

FIG. 4 is a simplified cross-sectional view of a window portion of theapparatus of FIG. 2, showing components of a laser interferometergenerating a laser beam and detecting a reflected interference beam.

FIG. 5 is a simplified cross-sectional view of a blank oxide wafer beingprocessed by the apparatus of FIG. 2, schematically showing the laserbeam impinging on the wafer and reflection beams forming a resultantinterference beam.

FIG. 6 is a simplified top view of the platen of the apparatus of FIG.2, showing one possible relative arrangement between the window andsensor flag, and the sensor and laser interferometer.

FIG. 7 is a top view of the platen of the apparatus of FIG. 2, showing arelative arrangement between the window and sensor flag, and the sensorand laser, where the window is in the shape of an arc.

FIG. 8 is a flow chart of a method of piece-wise data acquisition inaccordance with the present invention.

FIGS. 9A-B are graphs showing the cyclic variation in the data signalfrom the laser interferometer over time during the thinning of a blankoxide wafer. The graph of FIG. 9A shows the integrated values of thedata signal integrated over a desired sample time, and the graph of FIG.9B shows a filtered version of the integrated values.

FIG. 10A is a block diagram of a backward-looking method of determiningthe endpoint of a CMP process to thin the oxide layer of a blank oxidewafer in accordance with the present invention.

FIG. 10B is a block diagram of a forward-looking method of determiningthe endpoint of a CMP process to thin the oxide layer of a blank oxidewafer in accordance with the present invention.

FIGS. 11A-C are simplified cross-sectional views of a patterned waferwith an irregular surface being processed by the apparatus of FIG. 2,wherein FIG. 11A shows the wafer at the beginning of the CMP process,FIG. 11B shows the wafer about midway through the process, and FIG. 11Cshows the wafer close to the point of planarization.

FIG. 12 is a flow chart diagram of a method of determining the endpointof a CMP process to planarize a patterned wafer with an irregularsurface in accordance with the present invention.

FIG. 13 is a graph showing variation in the data signal from the laserinterferometer over time during the planarization of a patterned wafer.

FIG. 14 is a block diagram of a method of determining the endpoint of aCMP process to control the film thickness overlying a particularly sizedstructure, or group of similarly sized structures, in accordance withthe present invention.

FIG. 15A is a simplified cross-sectional view of a wafer with a surfaceimperfection being illuminated by a narrow-diameter laser beam.

FIG. 15B is a simplified cross-sectional view of a wafer with a surfaceimperfection being illuminated by a wide-diameter laser beam.

FIG. 16 is a graph showing the cyclic variation in the data signal fromthe laser interferometer over time during the thinning of a blank oxidewafer and including the high frequency signal associated with anonuniform wafer surface.

FIG. 17 is a schematic representation of a CMP system including aninterferometer and a computer programmed to analyze and respond to theoutput signal of interferometer waveform.

FIG. 18 is a block diagram showing the functionality that is implementedwithin the computer to perform in-situ monitoring of uniformity.

FIGS. 19( a)-(c) show examples of an interferometer signal, theinterferometer signal after it has been filtered by a low frequencybandpass pass filter, and the interferometer signal after it has beenfiltered by a high frequency bandpass pass filter, respectively.

FIG. 20( a)-(b) are flow charts showing the procedure for generating andthen using a signature of a CMP system to qualify it for production use.

FIG. 21( a) is simplified cross-sectional view of an embodiment of thewindow portion of the apparatus of FIG. 2 employing the polishing pad asthe window, and showing a reflection from the backside of the pad.

FIG. 21( b) is a graph showing the cyclical variation in the data signalfrom the laser interferometer over time with a large DC component causedby the reflection from the backside of the pad of the embodiment of FIG.21( a).

FIG. 21( c) is simplified cross-sectional view of an embodiment of thewindow portion of the apparatus of FIG. 2 employing the polishing pad asthe window with a diffused backside surface to suppress reflections.

FIG. 21( d) is a graph showing the cyclical variation in the data signalfrom the laser interferometer over time without the large DC componentcaused by reflection from the backside of the pad as a result of thediffuse backside surface of the embodiment of FIG. 21( c).

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 2 depicts a portion of a CMP apparatus modified in accordance withone embodiment of the present invention. A hole 30 is formed in theplaten 16 and the overlying platen pad 18. This hole 30 is positionedsuch that it has a view of the wafer 14 held by a polishing head 12during a portion of the platen's rotation, regardless of thetranslational position of the head 12. A laser interferometer 32 isfixed below the platen 16 in a position enabling a laser beam 34projected by the laser interferometer 32 to pass through the hole 30 inthe platen 16 and strike the surface of the overlying wafer 14 during atime when the hole 30 is adjacent the wafer 14.

A detailed view of the platen hole 30 and wafer 14 (at a time when itoverlies the platen hole 30) are shown in FIGS. 3A-C. As can be seen inFIG. 3A, the platen hole 30 has a stepped diameter, thus forming ashoulder 36. The shoulder 36 is used to contain and hold a quartz insert38 which functions as a window for the laser beam 34. The interfacebetween the platen 16 and the insert 38 is sealed, so that the portionof the chemical slurry 40 finding its way between the wafer 14 andinsert 38 cannot leak through to the bottom of the platen 16. The quartzinsert 38 protrudes above the top surface of the platen 16 and partiallyinto the platen pad 18. This protrusion of the insert 38 is intended tominimize the gap between the top surface of the insert 38 and thesurface of the wafer 14. By minimizing this gap, the amount of slurry 40trapped in the gap is minimized. This is advantageous because the slurry40 tends to scatter light traveling through it, thus attenuating thelaser beam emitted from the laser interferometer 32. The thinner thelayer of slurry 40 between the insert 38 and the wafer 14, the less thelaser beam 34 and light reflected from the wafer, is attenuated. It isbelieved a gap of approximately 1 mm would result in acceptableattenuation values during the CMP process. However, it is preferable tomake this gap even smaller. The gap should be made as small as possiblewhile still ensuring the insert 38 does not touch the wafer 14 at anytime during the CMP process. In a tested embodiment of the presentinvention, the gap between the insert 38 and wafer 14 was set at 10 mils(250 μm) with satisfactory results.

FIG. 3B shows an alternate embodiment of the platen 16 and pad 18. Inthis embodiment, the quartz insert has been eliminated and nothrough-hole exists in the pad 18. Instead, the backing layer 20 (ifpresent) of the pad 18 has been removed in the area overlying the hole30 in the platen 16. This leaves only the polyurethane covering layer 22of the pad 18 between the wafer 14 and the bottom of the platen 16. Ithas been found that the polyurethane material used in the covering layer22 will substantially transmit the laser beam 34 from the laserinterferometer 32. Thus, the portion of the covering layer 22 whichoverlies the platen hole 30 functions as a window for the laser beam 34.This alternate arrangement has significant advantages. First, becausethe pad 18 itself is used as the window, there is no appreciable gap.Therefore, very little of the slurry 40 is present to cause thedetrimental scattering of the laser beam. Another advantage of thisalternate embodiment is that pad wear becomes irrelevant. In thefirst-described embodiment of FIG. 3A, the gap between the quartz insert38 and the wafer 14 was made as small as possible. However, as the pad18 wears, this gap tends to become even smaller. Eventually, the wearcould become so great that the top surface of the insert 38 would touchthe wafer 14 and damage it. Since the pad 18 is used as the window inthe alternate embodiment of FIG. 3B, and is designed to be in contactwith the wafer 14, there are no detrimental effects due to the wearingof the pad 18. It is noted that tests using both the opaque open-celland transparent grooved surface types of pads have shown that the laserbeam is less attenuated with the transparent grooved surface pad.Accordingly, it is preferable that this type of pad be employed.

Although the polyurethane material used in the covering layer of the padis substantially transmissive to the laser beam, it does contain certainadditives, such as nylon microspheres, which inhibit itstransmissiveness. This problem is eliminated in the embodiment of theinvention depicted in FIG. 3C. In this embodiment, the typical padmaterial in the region overlying the platen hole 30 has been replacedwith a solid polyurethane plug 42. This plug 42, which functions as thewindow for the laser beam, is made of a polyurethane material whichlacks the nylon microspheres. Accordingly, the attenuation of the laserbeam 34 through the plug 42 is minimized. The plug 42 may be integrallymolded into the pad 18.

For example, the plug may be formed by pouring liquid polyurethane intoa hole that has been cut in the polishing pad. The liquid polyurethaneis cured to form a plug which is integrally molded into the polishingpad. Alternately, the plug 42 could be preformed as a solid insert. Thisinsert could be placed in the bulk molten polishing pad material, andthen the entire assembly could be cured so that the material of the plug42 and the material of the polishing pad 18 bond together. When theassembly is cooled, the polyurethane plug 42 would be integrally moldedinto the polishing pad. However, the material of the polishing pad 18,and specifically the covering layer 22, is different from the materialof the polyurethane plug 42. Therefore when the assembly is cured, thematerial of the plug 42 tends to contract and buckle the window up ordown. This causes either a cup which can accumulate slurry or a bumpwhich can damage the wafer 14.

Referring to FIG. 3D, in another embodiment, a two-level plug 600 ispositioned in the polishing pad 18 above the platen hole 30. Thetwo-level plug 600 is formed of a relatively transparent material whichacts as a window for the laser beam. The material of the two-level plug600 may be a substantially pure polyurethane available from Rodel ofNewark, N.J., under the product name EX-2000. Such a material ischemically inert vis-a-vis the polishing process, and erodes at the samerate as the polishing pad. The two-level plug 600 includes an upper plugportion 602 and a lower plug portion 604. The upper plug portion 602fits into a hole or opening 630 in the covering layer 22 and the lowerplug portion 604 fits into a hole or opening 632 in the backing layer20. The top surface 606 of the upper plug portion 602 is co-planar withthe top surface 23 of the polishing pad 18. There may be a gap 610between the lower surface 608 of the lower plug portion 604 and the topsurface 17 of the platen 16.

The application of a load from the wafer 14 on the polishing pad 18 willcause the backing layer 20 to compress. Thus, the width of the gap 610will decrease. The gap 610 is selected to be sufficiently wide that thelower surface 608 will not contact the upper surface 17 of the platen16, even if the wafer 14 is positioned over the platen hole 30. The topsurface 606 contacts the wafer 14 but, due to the gap 610, does notexert pressure on it. Therefore, the denser material of the two-levelplug 600 does not create a locally increased load. Thus, the two-levelplug 600 does not adversely affect the polishing of the wafer 14.

Referring to FIGS. 3E and 3F, the polishing pad 18 may be assembled asfollows. The two-level plug 600 is machined or molded from a solid pieceof polyurethane. An aperture 612 is cut into a polishing pad 18.Alternately, the polishing pad 18 may be integrally molded with theaperture 612. The aperture 612 includes two sections. The first sectionof the aperture may be the hole 630 in covering layer 22 and the secondsection of the aperture may be the hole 632 in the backing layer 20. Theaperture 612 matches the shape of two-level plug 600. The plug may be inthe form of adjacent rectangular slabs having different cross-sectionalareas. Specifically, the cross-sectional area of the lower plug portion604 may be larger than the cross-sectional area of the upper plugportion 602. The upper plug portion 602 may have a length L₁ of about2.0 inches and a height H₁ of about 0.5 inches The lower plug portion604 may have a length L₂ of about 2.2 inches and a height H₂ of about0.7 inches. Thus, the lower plug portion 604 extends beyond the upperplug portion 602 to form a rim 616 having a width W₁ of about 0.1inches. The plug may be oriented so that its longitudinal axis liesalong a radius of the polishing pad.

Although FIGS. 3D-F show the upper plug portion 602 as having a smallercross-sectional area than the lower plug portion 604, this is notnecessary. Instead, the upper plug portion 602 may be larger than thelower plug portion 604. The upper plug portion 602 has a thickness T₁equal to the thickness of covering layer 22, i.e., about fifty mils.Thus, the thickness T₁ is equal to the depth D₁ of the first section ofthe aperture. The lower plug portion 604 is thinner than the backinglayer 20 by about ten mils. The lower plug portion 604 may have athickness T₂ of about forty mils. Thus, the thickness T₂ is less thanthe depth D₂ of the second section of the aperture.

An adhesive material 614 is placed on the rim 616 of the lower plugportion 604. The adhesive material 614 may be an elastomericpolyurethane available from Berman Industries of Van Nuys, Calif. underthe trade name WC-575 A/B. Other adhesive materials, such as rubbercement or an epoxy, may also be used for the adhesive material 614.

An area 618 on the underside of covering layer 22 is cleaned by scrapingoff any adhesive debris and washing the area with acetone. Then thetwo-level plug 600 is inserted into the aperture 612 until the rim 616of plug 600 contacts the area 618 of the polishing pad 18. This contactarea is placed under a load of approximately fifteen to twenty poundsper square inch. This forces the adhesive material 614 into the gapsbetween upper plug portion 602 and covering layer 22 or between lowerplug portion 604 and backing layer 20. After a few days at roomtemperature, the adhesive material 614 will have cured and the plug 600will be fixed in the aperture 612. The adhesive material 614 could becured more quickly by the application of heat, but an excessivetemperature may deform the backing material 20.

There may be grooves or pores 620 cut into the covering layer 22 of thepolishing pad 18 to provide for improved slurry distribution. Thesegrooves or pores 620, which are located above the lower plug portion604, are filled with a pure polyurethane material 622. In addition, thetop surface 606 of the two-level plug 600 is left ungrooved. Becausethere are no grooves or depressions in the area of the two-level plug600, there is no accumulation of slurry which could block the laser beam34. During the conditioning process, in which a pad conditioner grindsaway the top surface 23 of the covering layer 22 to restore theroughness of the polishing pad 18, the top surface 606 of two-level plug600 will be scratched and abraded. Because polyurethane is a diffusivematerial, the abrasion of the top surface 606 will not significantlyaffect the performance of the laser interferometer 32.

The window provided by the two-level plug 600 prevents the accumulationof slurry above the platen hole 30 which could block the laser beam 34.The plug 600 is formed of a material which is chemically resistant tothe slurry 40 and is chemically inert vis-a-vis the polishing process.The plug erodes at the same rate as the rest of the polishing pad 18.The plug is sealed within the aperture to prevent the leakage of theslurry 40 into the platen hole 30, and the plug may be depressed toprevent the wafer from experiencing a locally increased load.

In operation, a CMP apparatus in accordance with the present inventionuses the laser beam from the laser interferometer to determine theamount of material removed from the surface of the wafer, or todetermine when the surface has become planarized. The beginning of thisprocess will be explained in reference to FIG. 4. It is noted that alaser and collimator 44, beam splitter 46, and detector 48 are depictedas elements of the laser interferometer 32. This is done to facilitatethe aforementioned explanation of the operation of the CMP apparatus. Inaddition, the embodiment of FIG. 3A employing the quartz insert 38 as awindow is shown for convenience. Of course, the depicted configurationis just one possible arrangement, others can be employed. For instance,any of the aforementioned window arrangements could be employed, andalternate embodiments of the laser interferometer 32 are possible. Onealternate interferometer arrangement would use a laser to produce a beamwhich is incident on the surface of the wafer at an angle. In thisembodiment, a detector would be positioned at a point where lightreflecting from the wafer would impinge upon it. No beam splitter wouldbe required in this alternate embodiment.

As illustrated in FIG. 4, the laser and collimator 44 generate acollimated laser beam 34 which is incident on the lower portion of thebeam splitter 46. A portion of the beam 34 propagates through the beamsplitter 46 and the quartz insert 38. Once this portion of beam 34leaves the upper end of the insert 38, it propagates through the slurry40, and impinges on the surface of the wafer 14. The wafer 14, as shownin detail in FIG. 5 has a substrate 50 made of silicon and an overlyingoxide layer 52 (i.e. SiO₂).

The portion of the beam 34 which impinges on the wafer 14 will bepartially reflected at the surface of the oxide layer 52 to form a firstreflected beam 54. However, a portion of the light will also betransmitted through the oxide layer 52 to form a transmitted beam 56which impinges on the underlying substrate 50. At least some of thelight from the transmitted beam 56 reaching the substrate 50 will bereflected back through the oxide layer 52 to form a second reflectedbeam 58. The first and second reflected beams 54, 58 interfere with eachother constructively or destructively depending on their phaserelationship, to form a resultant beam 60, where the phase relationshipis primarily a function of the thickness of the oxide layer 52.

Although, the above-described embodiment employs a silicon substratewith a single oxide layer, those skilled in the art will recognize theinterference process would also occur with other substrates and otheroxide layers. The key is that the oxide layer partially reflects andpartially transmits, and the substrate at least partially reflects, theimpinging beam. In addition, the interference process may also beapplicable to wafers with multiple layers overlying the substrate.Again, if each layer is partially reflective and partially transmissive,a resultant interference beam will be created, although it will be acombination of the reflected beams from all the layer and the substrate.

Referring again to FIG. 4, it can be seen the resultant beam 60representing the combination of the first and second reflected beams 54,58 (FIG. 5) propagates back through the slurry 40 and the insert 38, tothe upper portion of the beam splitter 46. The beam splitter 46 divertsa portion of the resultant beam 60 towards the detector 48.

The platen 16 will typically be rotating during the CMP process.Therefore, the platen hole 30 will only have a view of the wafer 14during part of its rotation. Accordingly, the detection signal from thelaser interferometer 32 should only be sampled when the wafer 14 isimpinged by the laser beam 34. It is important that the detection signalnot be sampled when the laser beam 34 is partially transmitted throughthe hole 30, as when a portion is blocked by the bottom of the platen 16at the hole's edge, because this will cause considerable noise in thesignal. To prevent this from happening, a position sensor apparatus hasbeen incorporated. Any well known proximity sensor could be used, suchas Hall effect, eddy current, optical interrupter, or acoustic sensor,although an optical interrupter type sensor was used in the testedembodiments of the invention and will be shown in the figures thatfollow. An apparatus accordingly to the present invention forsynchronizing the laser interferometer 32 is shown in FIG. 6, with anoptical interrupter type sensor 62 (e.g. LED/photodiode pair) mounted ona fixed point on the chassis of the CMP device such that it has a viewof the peripheral edge of the platen 16. This type of sensor 62 isactivated when an optical beam it generates is interrupted. A positionsensor flag 64 is attached to the periphery of the platen 16. The pointof attachment and length of the flag 64 is made such that it interruptsthe sensor's optical signal only when the laser beam 34 from the laserinterferometer 32 is completely transmitted through thepreviously-described window structure 66. For example, as shown in FIG.6, the sensor 62 could be mounted diametrically opposite the laserinterferometer 32 in relation to the center of the platen 16. The flag64 would be attached to the platen 16 in a position diametricallyopposite the window structure 66. The length of the flag 64 would beapproximately defined by the dotted lines 68, although, the exact lengthof the flag 64 would be fine tuned to ensure the laser beam iscompletely unblocked by the platen 16 during the entire time the flag 64is sensed by the sensor 62. This fine tuning would compensate for anyposition sensor noise or inaccuracy, the responsiveness of the laserinterferometer 32, etc. Once the sensor 62 has been activated, a signalis generated which is used to determine when the detector signal fromthe interferometer 32 is to be sampled.

Data acquisition systems capable of using the position sensor signal tosample the laser interferometer signal during those times when the waferis visible to the laser beam, are well known in the art and do not forma novel part of the present invention. Accordingly, a detaileddescription will not be given herein. However some considerations shouldbe taken into account in choosing an appropriate system. For example, itis preferred that the signal from the interferometer be integrated overa period of time. This integration improves the signal-to-noise ratio byaveraging the high frequency noise over the integration period. Thisnoise has various causes, such as vibration from the rotation of theplaten and wafer, and variations in the surface of the wafer due tounequal planarization. In the apparatus described above the diameter ofthe quartz window, and the speed of rotation of the platen, willdetermine how long a period of time is available during any one rotationof the platen to integrate the signal. However, under somecircumstances, this available time may not be adequate. For instance, anacceptable signal-to-noise ratio might require a longer integrationtime, or the interface circuitry employed in a chosen data acquisitionsystem may require a minimum integration time which exceeds that whichis available in one pass.

One solution to this problem is to extend the platen hole along thedirection of rotation of the platen. In other words, the windowstructure 66′ (i.e. insert, pad, or plug) would take on the shape of anarc, as shown in FIG. 7. Of course, the flag 64′ is expanded toaccommodate the longer window structure 66′. Alternately, the windowcould remain the same, but the laser interferometer would be mounted tothe rotating platen directly below the window. In this case, the CMPapparatus would have to be modified to accommodate the interferometerbelow the platen, and provisions would have to be made to route thedetector signal from the interferometer. However, the net result ofeither method would be to lengthen the data acquisition time for eachrevolution of the platen.

Although lengthening the platen hole and window is advantageous, it doessomewhat reduce the surface area of the platen pad. Therefore, the rateof planarization is decreased in the areas of the disk which overlie thewindow during a portion of the platen's rotation. In addition, thelength of the platen hole and window must not extend beyond the edges ofthe wafer, and the data sampling must not be done when the window isbeyond the edge of the wafer, regardless of the wafer's translationalposition. Therefore, the length of the expanded platen hole and window,or the time which the platen-mounted interferometer can be sampled, islimited by any translational movement of the polishing head.

Accordingly, a more preferred method of obtaining adequate dataacquisition integration time is to collect the data over more than onerevolution of the platen. In reference to FIG. 8, during step 102, thelaser interferometer signal is sampled during the available dataacquisition time in each rotation of the platen. Next, in steps 104 and106, each sampled signal is integrated over the aforementioned dataacquisition time, and the integrated values are stored. Then, in steps108 and 110, a cumulative sample time is computed after each completerevolution of the platen and compared to a desired minimum sample time.Of course, this would constitute only one sample time if only one samplehas been taken. If the cumulative sample time equals or exceeds thedesired minimum sample time, then the stored integrated values aretransferred and summed, as shown in step 112. If not, the process ofsampling, integrating, storing, computing the cumulative sample time,and comparing it to the desired minimum sample time continues. In afinal step 114, the summed integrated values created each time thestored integrated values are transferred and summed, are output as adata signal. The just-described data collection method can beimplemented in a number of well known ways, employing either logiccircuits or software algorithms. As these methods are well known, anydetailed description would be redundant and so has been omitted. It isnoted that the method of piece-wise data collection provides a solutionto the problem of meeting a desired minimum sample time no matter whatthe diameter of the window or the speed of platen rotation. In fact, ifthe process is tied to the position sensor apparatus, the platenrotation speed could be varied and reliable data would still beobtained. Only the number of platen revolutions required to obtain thenecessary data would change.

The aforementioned first and second reflected beams which formed theresultant beam 60, as shown in FIGS. 4 and 5, cause interference to beseen at the detector 48. If the first and second beams are in phase witheach other, they cause a maxima on detector 48. Whereas, if the beamsare 180 degrees out of phase, they cause a minima on the detector 48.Any other phase relationship between the reflected beams will result inan interference signal between the maxima and minima being seen by thedetector 48. The result is a signal output from the detector 48 thatcyclically varies with the thickness of the oxide layer 52, as it isreduced during the CMP process. In fact, it has been observed that thesignal output from the detector 48 will vary in a sinusoidal-likemanner, as shown in the graphs of FIGS. 9A-B. The graph of FIG. 9A showsthe integrated amplitude of the detector signal (y-axis) over eachsample period versus time (x-axis). This data was obtained by monitoringthe laser interferometer output of the apparatus of FIG. 4, whileperforming the CMP procedure on a wafer having a smooth oxide layeroverlying a silicon substrate (i.e. a blank oxide wafer). The graph ofFIG. 9B represents a filtered version of the data from the graph of FIG.9A. This filtered version shows the cyclical variation in theinterferometer output signal quite clearly. It should be noted that theperiod of the interference signal is controlled by the rate at whichmaterial is removed from the oxide layer during the CMP process. Thus,factors such as the downward force placed on the wafer against theplaten pad, and the relative velocity between the platen and the waferdetermine the period. During each period of the output signal plotted inFIGS. 9A-B, a certain thickness of the oxide layer is removed. Thethickness removed is proportional to the wavelength of the laser beamand the index of refraction of the oxide layer. Specifically, the amountof thickness removed per period is approximately λ/2n, where λ is thefreespace wavelength of the laser beam and n is the index of refractionof the oxide layer. Thus, it is possible to determine how much of theoxide layer is removed, in-situ, during the CMP process using the methodillustrated in FIG. 10A. First, in step 202, the number of cyclesexhibited by the data signal are counted. Next, in step 204, thethickness of the material removed during one cycle of the output signalis computed from the wavelength of the laser beam and the index ofrefraction of the oxide layer of the wafer. Then, the desired thicknessof material to be removed from the oxide layer is compared to the actualthickness removed, in step 206. The actual thickness removed equals theproduct of the number of cycles exhibited by the data signal and thethickness of material removed during one cycle. In the final step 208,the CMP process is terminated whenever the removed thickness equals orexceeds the desired thickness of material to be removed.

Alternately, less than an entire cycle might be used to determine theamount of material removed. In this way any excess material removed overthe desired amount can be minimized. As shown in the bracketed portionsof the step 202 in FIG. 10A, the number of occurrences of a prescribedportion of a cycle are counted in each iteration. For example, eachoccurrence of a maxima (i.e. peak) and minima (i.e. valley), or viceversa, would constitute the prescribed portion of the cycle. Thisparticular portion of the cycle is convenient as maxima and minima arereadily detectable via well know signal processing methods. Next, instep 204, after determining how much material is removed during a cycle,this thickness is multiplied by the fraction of a cycle that theaforementioned prescribed portion represents. For example in the case ofcounting the occurrence of a maxima and minima, which representsone-half of a cycle, the computed one-cycle thickness would bemultiplied by one-half to obtain the thickness of the oxide layerremoved during the prescribed portion of the cycle. The remaining stepsin the method remain unchanged. The net result of this alternateapproach is that the CMP process can be terminated after the occurrenceof a portion of the cycle. Accordingly, any excess material removedwill, in most cases, be less than it would have been if a full cyclewhere used as the basis for determining the amount of material removed.

The just-described methods look back from the end of a cycle, or portionthereof, to determine if the desired amount of material has beenremoved. However, as inferred above, the amount of material removedmight exceed the desired amount. In some applications, this excessremoval of material might be unacceptable. In these cases, an alternatemethod can be employed which looks forward and anticipates how muchmaterial will be removed over an upcoming period of time and terminatesthe procedure when the desired thickness is anticipated to have beenremoved. A preferred embodiment of this alternate method is illustratedin FIG. 10B. As can be seen, the first step 302 involves measuring thetime between the first occurrence of a maxima and minima, or vice versa,in the detector signal (although an entire cycle or any portion thereofcould have been employed). Next, in step 304, the amount of materialremoved during that portion of the cycle is determined via thepreviously described methods. A removal rate is then calculated bydividing the amount of material removed by the measured time, as shownin step 306. This constitutes the rate at which material was removed inthe preceding portion of the cycle. In the next step 308, the thicknessof the material removed as calculated in step 304 is subtracted from thedesired thickness to be removed to determine a remaining removalthickness. Then, in step 310, this remaining removal thickness isdivided by the removal rate to determine how much longer the CMP processis to be continued before its termination.

It must be noted, however, that the period of the detector signal, andso the removal rate, will typically vary as the CMP process progresses.Therefore, the above-described method is repeated to compensate forthis. In other words, once a remaining time has been calculated, theprocess is repeated for each occurrence of a maxima and minima, or viceversa. Accordingly, the time between the next occurring maxima andminima is measured, the thickness of material removed during the portionof the cycle represented by this occurrence of the maxima and minima(i.e. one-half) is divided by the measured time, and the removal rate iscalculated, just as in the first iteration of the method. However, inthe next step 308, as shown in brackets, the total amount of materialremoved during all the previous iterations is determined before beingsubtracted from the desired thickness. The rest of the method remainsthe same in that the remaining thickness to be removed is divided by thenewly calculated removal rate to determine the remaining CMP processtime. In this way the remaining process time is recalculated after eachoccurrence of the prescribed portion of a cycle of the detector signal.This process continues until the remaining CMP process time will expirebefore the next iteration can begin. At that point the CMP process isterminated, as seen in step 312. Typically, the thickness to be removedwill not be accomplished in the first one-half cycle of the detectorsignal, and any variation in the removal rate after being calculated forthe preceding one-half cycle will be small. Accordingly, it is believedthis forward-looking method will provide a very accurate way of removingjust the desired thickness from the wafer.

While the just-described monitoring procedure works well for thesmooth-surfaced blank oxide wafers being thinned, it has been found thatthe procedure cannot be successfully used to planarize most patternedwafers where the surface topography is highly irregular. The reason forthis is that a typical patterned wafer contains dies which exhibit awide variety of differently sized surface features. These differentlysized surface features tend to polish at different rates. For example, asmaller surface feature located relatively far from other features tendsto be reduced faster than other larger features. FIG. 11A-C exemplify aset of surface features 72, 74, 76 of the oxide layer 52 associated withunderlying structures 78, 80, 82, that might be found on a typicalpatterned wafer 14, and the changes they undergo during the CMP process.Feature 72 is a relatively small feature, feature 74 is a medium sizedfeature, and feature 76 is a relatively large feature. FIG. 11A showsthe features 72, 74, 76 before polishing, FIG. 11B shows the features72, 74, 76 about midway through the polishing process, and FIG. 11Cshows the features 72, 74, 76 towards the end of the polishing process.In FIG. 11A, the smaller feature 72 will be reduced at a faster ratethan either the medium or large features 74, 76. In addition, the mediumfeature 74 will be reduced at a faster rate than the large feature 76.The rate at which the features 72, 74, 76 are reduced also decreases asthe polishing process progresses. For example, the smaller feature 72will initially have a high rate of reduction, but this rate will dropoff during the polishing process. Accordingly, FIG. 11B shows the heightof the features 72, 74, 76 starting to even out, and FIG. 11C shows theheight of the features 72, 74, 76 essentially even. Since thedifferently sized features are reduced at different rates and theserates are changing, the interference signal produced from each featurewill have a different phase and frequency. Accordingly, the resultantinterference signal, which is partially made up of all the individualreflections from each of the features 72, 74, 76, will fluctuate in aseemingly random fashion, rather than the previously described periodicsinusoidal signal.

However, as alluded to above, the polishing rates of the features 72,74, 76 tend to converge closer to the point of planarization. Therefore,the difference in phase and frequency between the interference beamsproduced by the features 72, 74, 76 tend to approach zero. This resultsin the resultant interference signal becoming recognizable as a periodicsinusoidal wave form. Therefore, it is possible to determine when thesurface of a patterned wafer has become planarized by detecting when asinusoidal interference signal begins. This method is illustrated inFIG. 12. First, in step 402, a search is made for the aforementionedsinusoidal variation in the interferometer signal. When the sinusoidalvariation is discovered, the CMP procedure is terminated, as shown instep 404.

FIG. 13 is a graph plotting the amplitude of the detector signal overtime for a patterned wafer undergoing a CMP procedure. The sampled dataused to construct this graph was held at its previous integrated valueuntil the next value was reported, thus explaining the squared-off peakvalues shown. A close inspection shows that a discernible sinusoidalcycle begins to emerge at approximately 250 seconds. This coincides withthe point where the patterned wafer first became planarized. Of course,in real-time monitoring of the interferometer's output signal, it wouldbe impossible to know exactly when the cycling begins. Rather, at leastsome portion of the cycle must have occurred before it can be certainthat the cycling has begun. Preferably, no more than one cycle isallowed to pass before the CMP procedure is terminated. A one-cyclelimit is a practical choice because it provides a high confidence thatthe cycling has actually begun, rather than the signal simplyrepresenting variations in the noise caused by the polishing of thedifferently sized features on the surface of the wafer. In addition, theone-cycle limit ensures only a small amount of material is removed fromthe surface of the wafer after it becomes planarized. It has been foundthat the degree of planarization is essentially the same after twocycles, as it was after one. Thus, allowing the CMP procedure tocontinue would only serve to remove more material from the surface ofthe wafer. Even though one cycle is preferred in the case where the CMPprocess is to be terminated once the patterned wafer becomes planarized,it is not intended that the present invention be limited to that timeframe. If the signal is particularly strong, it might be possible toobtain the same level of confidence after only a portion of a cycle.Alternately, if the signal is particularly weak, it may take more thanone cycle to obtain the necessary confidence. The choice will depend onthe characteristics of the system used. For instance, the size of thegap between the quartz window and the surface of the wafer will have aneffect on signal strength, and so the decision on how many cycles towait before terminating the CMP process.

The actual determination as to when the output signal from the laserinterferometer is actually cycling, and so indicating that the surfaceof the wafer has been planarized can be done in a variety of ways. Forexample, the signal could be digitally processed and an algorithmemployed to make the determination. Such a method is disclosed in U.S.Pat. No. 5,097,430, where the slope of the signal is used to make thedetermination. In addition, various well known curve fitting algorithmsare available. These methods would essentially be used to compare theinterferometer signal to a sinusoidal curve. When a match occurs withinsome predetermined tolerance, it is determined that the cycling hasbegun. Some semiconductor applications require that the thickness of thematerial overlying a structure formed on a die of a patterned wafer(i.e. the film thickness) be at a certain depth, and that this filmthickness be repeatable from die to die, and from wafer to wafer. Thepreviously described methods for planarizing a typical patterned waferwill not necessarily produce this desired repeatable film thickness. Thepurpose of the planarization methods is to create a smooth and flatsurface, not to produce a particular film thickness. Accordingly, if itis desirable to control the film thickness over a specific structure, orgroup of similarly sized structures, an alternate method must beemployed. This alternate method is described below.

As alluded to previously, each differently sized surface featureresulting from a layer of oxide being formed over a patterned structureon a die tends to produce a reflected interference signal with a uniquefrequency and phase. It is only close to the point of planarization thatthe frequency and phase of each differently sized feature converges.Prior to this convergence the unique frequency and phase of theinterference signals caused by the various differently sized featurescombine to produce a detector signal that seems to vary randomly.However, it is possible to process this signal to eliminate theinterference signal contributions of all the features being polished atdifferent rates, except a particularly sized feature, or group ofsimilarly sized features. Once the interference signal associated withthe particularly sized feature, or group of features, has been isolated,the methods discussed in association with the removal of material from ablank oxide disk are employed to remove just the amount of materialnecessary to obtain the desired film thickness.

Of course, the frequency of the interference signal component caused bythe feature of interest must be determined prior to the signalprocessing. It is believed this frequency can be easily determined byperforming a CMP process on a test specimen which includes diesexclusively patterned with structures corresponding to the structurewhich is to have a particular overlying film thickness. The detectorsignal produced during this CMP process is analyzed via well knownmethods to determine the unique frequency of the interference signalcaused by the surface features associated with the aforementionedstructures.

The specific steps necessary to perform the above-described method ofcontrolling the film thickness over a specific structure, or group ofsimilarly sized structures on a die, in situ, during the CMP processingof a wafer, will now be described in reference to FIG. 14. In step 502,the detector signal is filtered to pass only the component of the signalhaving the predetermined frequency associated with the structure ofinterest. This step is accomplished using well known band pass filteringtechniques. Next, in step 504 a measurement is made of the time betweenthe first occurrence of a maxima and minima, or vice versa, in thedetector signal (although an entire cycle or any portion thereof couldhave been employed). The amount of material removed during that portionof the cycle (i.e. one-half cycle) is determined in step 506 viapreviously described methods. Then, a removal rate is then calculated bydividing the amount of material removed by the measured time, as shownin step 508. This constitutes the rate at which material was removed inthe preceding portion of the cycle. In the next step 510, the thicknessof the material removed as calculated in step 506 is subtracted from thedesired thickness to be removed (i.e. the thickness which when removedwill result in the desired film thickness overlying the structure ofinterest), to determine a remaining removal thickness. Then, thisremaining removal thickness is divided by the aforementioned removalrate to determine how much longer the CMP process is to be continuedbefore it termination, in step 512. Once a remaining time has beencalculated, the process is repeated for each occurrence of a maxima andminima, or vice versa. Accordingly, the time between the next occurringmaxima and minima is measured, the thickness of material removed duringthe portion of the cycle represented by this occurrence of the maximaand minima (i.e. one-half) is divided by the measured time, and theremoval rate is calculated, just as in the first iteration of themethod. However, in the next step 510, as shown in brackets, the totalamount of material removed during all the previous iterations isdetermined before being subtracted from the desired thickness. The restof the method remains the same in that the remaining thickness to beremoved is divided by the newly calculated removal rate to determine theremaining CMP process time. This process is repeated until the remainingtime expires before the next iteration can begin. At that point, the CMPprocess is terminated, as seen in step 514.

It is noted that although the method for controlling film thicknessdescribed above utilizes the method for determining the CMP processendpoint illustrated in FIG. 10B, any of the other endpointdetermination methods described herein could also be employed, ifdesired.

It is further noted that the beam diameter (i.e. spot) and wavelength ofthe laser beam generated by the laser interferometer can beadvantageously manipulated. As shown in FIGS. 15A and 15B, a narrow beam84, such as one focused to the smallest spot possible for the wavelengthemployed, covers a smaller area of the surface of the wafer 14 than awider, less focused beam 86. This narrow beam 84 is more susceptible toscattering (i.e. beam 88) due to surface irregularities 90, than thewider beam 86, since the wider beam 86 spreads out over more of thesurface area of the wafer 14, and encompasses more of the surfaceirregularities 90. Therefore, a wider beam 86 would have an integratingeffect and would be less susceptible to extreme variations in thereflected interference signal, as it travels across the surface of thewafer 14. Accordingly, a wider beam 86 is preferred for this reason. Thelaser beam width can be widened using well known optical devices.

It must also be pointed out that the wider beam will reduce theavailable data acquisition time per platen revolution since the time inwhich the beam is completely contained within the boundaries of thewindow is less than it would be with a narrower beam. However, with thepreviously described methods of data acquisition, this should notpresent a significant problem. In addition, since the wider beam alsospreads the light energy out over a larger area than a narrower beam,the intensity of the reflections will be lessen somewhat. This drawbackcan be remedied by increasing the power of the laser beam from the laserinterferometer so that the loss in intensity of the reflected beams isnot a factor in detection.

As for the wavelength of the laser beam, it is feasible to employ awavelength anywhere from the far infrared to ultraviolet. However, it ispreferred that a beam in the red light range be used. The reason forthis preference is two-fold. First, shorter wavelengths result in anincrease in the amount of scattering caused by the chemical slurrybecause this scattering is proportional to the 4th power of thefrequency of the laser beam. Therefore, the longer the wavelength, theless the scattering. However, longer wavelengths also result in more ofthe oxide layer being removed per period of the interference signal,because the amount of material removed per period equals approximatelyλ/2n. Therefore, the shorter the wavelength, the less material removedin one period. It is desirable to remove as little of the material aspossible during each period so that the possibility of any excessmaterial being removed is minimized. For example, in a system employingthe previously described method by which the number of cycles, or aportion thereof, are counted to determine the thickness of the oxidelayer removed, any excess material removed over the desired amount wouldbe minimized if the amount of material removed during each cycle, orportion thereof, is as small as possible.

It is believed these two competing factors in the choice of wavelengthare optimally balance if a red light laser beam is chosen. Red lightoffers an acceptable degree of scattering while not resulting in anunmanageable amount of material being removed per cycle.

FURTHER EMBODIMENTS

The generated interference waveform provides considerable additionalinformation about the polishing process. This additional information canbe used to provide an in-situ measurement of the uniformity of thepolished layer. It can also be used to detect when the CMP system is notoperating within spec (i.e., not operating as desired). Both of theseuses will now be described.

Uniformity Measurement:

The polishing and/or planarization operations which are performed on theCMP system are generally required to produce a surface layer that isuniform across the surface of the wafer/substrate. In other words, thecenter of the wafer should polish at the same rate as the edge of thewafer. Typically, the thickness of the polished layer must not vary bymore than about 5-10%. If that level of uniformity is not achieved, itis likely that the wafer will not be usable since the device yields willbe unacceptably low. In practice, it is often quite difficult to achievea uniform polishing rate across the wafer. It typically requiresoptimizing many different variables to keep it performing within thespecs. The end point detector described above provides a very usefultool for monitoring the uniformity of the layer being polished and thatmonitoring can be performed both in-situ data acquisition andprocessing.

We have discovered that the interference waveform that is produced bythe interferometer during polishing provides information about theuniformity of the layer that is being polished. As noted above, theoutput of the interferometer appear as a sinusoidal signal as thesurface layer (e.g. oxide layer) is being polished. The distance betweenthe peaks of that signal indicate how much material has been removed. Ontop of that sinusoidal signal there will also be another higherfrequency sinusoidal signal. The amplitude of the higher frequencysignal indicates by how much the thickness of the polished layer variesacross the surface of the wafer.

The reason that the high frequency signal appears is as follows. As thepolishing is being performed, the interferometer typically samples (orlooks at) different locations across the surface of the wafer. This isbecause during polishing, both the platen and the wafer are rotating andin addition the wafer is also being moved axially relative to theplaten. Thus, during polishing different areas of the wafer's surfacepass over the hole in the platen through which the interferometer seesthe layer that is being polished. If the polished layer is completelyuniform, the resulting interference waveform will be unaffected by thesampling of the different locations across the wafer's surface. That is,it will have substantially the same amplitude. On the other hand, if thepolished layer is not uniform, the sampling of different locationsintroduce, a further variation onto the sinusoidal base signal. Thisfurther variation has a frequency that is dependent on the rotation andsweep rates that are used and it has an amplitude that is proportionalto the degree of nonuniformity of the polished layer. An example of sucha waveform is shown in FIG. 16. In this particular example, thenonuniformity was relatively large so as to clearly illustrate the highfrequency signal.

A measure of the uniformity is the ratio of the peak-to-peak amplitudeA_(hf) of the high frequency signal to the peak-to-peak amplitude A_(lf)of the low frequency signal. The smaller this ratio, the more uniformthe polished layer will be; and conversely, the larger this ratio, themore nonuniform it will be.

A CMP system which produces a measure of uniformity is shown in FIG. 17.In addition to the components shown in the previously described FIG. 2,it also includes a computer 150, which is programmed to control theoperation of the interferometer and to perform the signal analysis thatis required to produce a measure of uniformity from the interferencesignal, and it includes a display unit 160 through which variousinformation and results are displayed to an operator. Computer 150 canbe any device which is capable of performing the control and signalprocessing functions including, for example, a standard PC which isprogrammed appropriately and a dedicated, specially designed digitalprocessing unit. Display unit 160 can be a video display, a printer, orany appropriate device or combination of devices for communicatinginformation to the operator of the CMP system.

To generate a uniformity measure, computer 150 is programmed toimplement and perform the signal processing and other functions shown inFIG. 18. In that regard, computer 150 implements two programmablebandpass filters, namely, a high frequency filter 152 and a lowfrequency filter 154. High frequency filter 152 has a passband centeredon the frequency of the high frequency signal containing the uniformityinformation and low frequency filter 154 has a passband centered on thefrequency of the low frequency signal containing the polishing rateinformation. The width of both of these passbands is on the order of afew milliherz in the case when the period is on the order of tens ofseconds. Indeed, the width of the passband is programmed to vary inproportion with the center frequency, or stated differently, to varyinversely to the period of the signal being examined. That is, if theperiod of the relevant signal increases, the bandwidth of the passbandfilter decreases and vice versa.

FIG. 19( a) shows an example of an interferometer signal obtain from anactual system. Note that initially the signal indicates that the layeris quite uniform, i.e., no discernible high frequency signal is ridingon top of the low frequency signal. After polishing has been performedfor a short period of time, a high frequency signal begins to appear,indicating a certain level of nonuniformity. Low frequency filter 154selects the low frequency component and filters out the otherfrequencies to produce an output signal of the form shown in FIG. 19(b). Similarly, high frequency filter 152 selects the high frequencycomponent and filters out the other frequencies to produce an outputsignal of the form shown in FIG. 19( c).

Computer 150 implements two amplitude measurement functions 156 and 158which measure the peak-to-peak amplitudes of the output signals offilters 152 and 154, respectively. Once the amplitudes of the twofiltered signals has been determined, computer 150 computes a ratio ofthe p-p amplitude of the high frequency signal to the p-p amplitude ofthe low frequency signal (i.e., A_(hf)/A_(lf)) (see functional block162). After the ratio has been computed, computer 150 compares (seeblock 166) the computed ratio to a threshold or reference value 164 thatwas previously stored in local memory. If the computed ratio exceeds thestored threshold value, computer 150 alerts the operator thatnonuniformity of the polished layer exceeds an acceptable amount. Inresponse, the operator can adjust the process parameters to bring theprocess back into spec.

Since the high frequency signal tends to appear only after somepolishing has been performed, it is useful to wait before attempting tomeasure nonuniformity. Indeed, it may be desirable to automaticallycompute the ratio periodically so as to monitor the uniformity of thepolished layer throughout the polishing operation. In that case, it mayalso be desirable for computer 150 to output the computed ratiosthroughout the process so that the operator can detect changes and/ortrends which are appearing in the polishing process. This would beparticularly useful if the in-situ monitoring was done during on actualproduction wafers during polishing.

Note that the functions just described can be implemented throughsoftware that is running on the computer or they can be implementedthrough dedicated circuits built for this specific purpose.

The bandpass filters can be implemented using techniques which are wellknown to persons skilled in the art. In the described embodiment, theyare FIR (finite impulse response) filters which can be implemented ineither the frequency or the time domain. However, to perform thefiltering in real time as the interferometer signal becomes available,the filtering is done in the time domain by convolving the appropriatefunction with the waveform as it is being generated. The appropriatefunction is, of course, simply the time domain representation of abandpass filter having the desired characteristics (i.e., centerfrequency and bandwidth).

To specify the appropriate filter parameters it is necessary to know thefrequency of the signal that is to be selected by the filter. Thisinformation can be obtained easily from the interferometer signalwaveform(s). For example, the center frequency for the low frequencyfilter can be obtained by running a batch (e.g. 25) of wafers (e.g.blank wafers with only an oxide coating) to obtain an accurate measureof the polishing rate. Alternatively, the polishing rate can bedetermined at the start of a polishing run by measuring the distancebetween peaks of the low frequency signal. Of course, using thisalternative approach produces results that are not as accurate asaveraging measurements over a larger number of wafers. In any case, thepolishing rate determines the center frequency of the bandpass filterand by knowing the center frequency along with the desired bandwidth ofthe filter one can readily determine the precise form of the time domainfilter function and/or the coefficients of the FIR filter.

The frequency of the high frequency signal can be obtained in a similarmanner, i.e., directly from the trace that is generated by theinterferometer as the CMP system is polishing the wafer. In other words,the operator simply measures the distance between peaks of the highfrequency signal. This process can be readily automated so that theoperator, with the aid of a pointing device (e.g. a mouse), can mark twopoints on the waveform appearing on a video display and the computer canbe programmed to automatically compute the frequency and then generatethe appropriate filter coefficients. The filter coefficients and/or timedomain representation of the filter functions are then stored in localmemory for use later during the polishing runs to perform the filteringoperations.

Process Signature:

The interferometer waveform also represents a signature of (i.e., itcharacterizes) the system for which it was obtained. Because of this, itprovides information which is useful for qualifying a system forproduction operation. If a signature is obtained for a system that isknown to be operating as desired, that signature waveform (or featuresextracted from the waveform) can be used as a reference against whichsubsequently generated signatures can be compared to determine whetherthe system or systems from which signatures were subsequently obtainedare performing within spec. For example, if the polishing pads arechanged or a new batch of slurry is used in the CMP system, the operatorneeds to know whether that change has detrimentally affected the qualityof the polishing which the system performs. We have discovered that achange in performance of the CMP system results in a change in thesignature. That is, certain features will appear in the waveform thatwere not previously present or previously existing features will change.By detecting those changes, it is possible to detect when a system isnot performing as desired.

In the described embodiment, the extracted features from theinterferometer waveform are the polishing rate and the measure ofuniformity. Both of these characteristics are readily obtainable fromthe interferometer waveform that is generated during polishing by usingthe methods previously described. A properly operating system willproduce a particular polishing rate and a particular measure ofuniformity. A drift away from these reference values provides anindication that the system is moving away from its desired operatingpoint and alerts the operator to the need for corrective action so as toavoid destroying product.

A method which uses a CMP system signature is illustrated in FIG. 20 aand will now be described. Initially, an interferometer waveform (i.e.,a signature) is generated for a CMP system which is known to beoperating optimally (step 250). The decision as to whether the system isoperating optimally can be determined empirically by processing a set oftest wafers and analyzing the results. When the results that areproduced are within spec, then the signature can be generated for thatconfiguration and set of operating conditions. Before capturing aportion of the interferometer waveform, it is desirable to polish thewafer between 50-100% of the way through the oxide so that the waveformis truly a signature of the polishing set up.

After the waveform has been obtained, certain relevant features are thenextracted from the generated waveform (step 252) and stored for lateruse as a reference against which to evaluate that system's performanceat some later time or times (step 254). Alternatively, the waveformitself can be stored and used as the reference. In the describedembodiment, the extracted features are the polishing rate and themeasure of uniformity, both of which can be determined from the waveformas described above.

Referring to FIG. 20 b, at some later time the stored signature (orextracted features) can be used to qualify that system or another systemfor production use. To qualify a system for production, a new signatureis obtained for that system (step 258) and the relevant features areextracted from that new signature (step 260). The extracted features arethen compared to the stored reference set of features (step 264). If theoperating point, as characterized by the set of extracted features,falls within a predetermined region around the reference point, asdefined by the stored reference set of features, then it is concludedthat the system is operating properly and that it can be brought onlinefor processing product wafers (step 266). If this process is automated,the computer may at this point alert the operator that the process iswithin spec. On the other hand, if the operating point falls outside ofthe predetermined region, that is an indication that the system is notoperating within spec and the operator is alerted to this problem sothat corrective action can be taken (step 268). The corrective actionmight involve adjusting some process parameter appropriately to bringthe process within spec. For example, if the polishing rate is excessiveor if oxide nonuniformity is larger than permitted, then the operatormay recognize that it is appropriate to try a new batch of slurry, or toadjust the pressure on the pad, or to even replace the pad. Theparticular course of corrective action that is chosen will of coursedepend upon the details of how the system has departed from its desiredoperating point, the configuration and operating parameters of theparticular system, and what the operator's experience has taught him.

To provide further useful information to the operator, the computer alsooptionally outputs through its display device(s) information about theextracted features (step 262). The displayed information may bepresented as the extracted features, the waveform, how close the variousextracted features are to the different features of the stored referenceset, or in whatever manner proves to be most useful for the operator.

Of course, the above-described in-situ, real time monitoring procedurecan be used periodically while processing production wafers or wheneversome process parameter is changed in the CMP system (e.g. a newpolishing pad is used, pad pressure is adjusted, or a new batch ofslurry is used) and it becomes necessary to know that the CMP process isstill within spec. In addition, it can be used on blank wafers, insteadof actual product, to qualify the CMP system prior to using it on actualproduct.

Though we have described a straight forward and simple approach toextracting information from the signature waveform, i.e., by using thepolishing rate and the measure of uniformity, the signature orinterferometer waveform can be analyzed by using more sophisticatedtechniques (e.g. pattern or feature recognition or other image analysisalgorithms, or neural networks, just to name a few alternatives). Theinformation which various extracted features convey regarding theoperation of the system can be determined through experience and theones which convey the information that is perceived to be of mostimportance to the operator can be used.

Also, it should be noted that simply displaying the interferometerwaveform (i.e., the process signature) to the operator can be yieldvaluable feedback on how well the system is behaving. Typically, thehuman eye is extremely sensitive in detecting even subtle changes in animage from what one expects to see. Thus, after gaining some experience,the operator will often be able to detect changes and imminent problemsin the overall CMP system performance simply by looking at the waveform.Thus, in the described embodiment, the computer also displays thesignature waveform to the operator during processing so that theoperator can also use it to monitor equipment performance.

Using techniques known to persons skilled in the art, one can readilydevelop software algorithms which automatically recognize or detect thechanges for which the operator is looking and which tip off the operatorto certain problems.

A Modification for Obtaining Improved Performance

Another embodiment involves a modification to the window in the padbetween the interferometer and the wafer. Although the pad will transmita substantial portion of the interferometer laser beam, it has beenfound that there is also a significant reflective component from thebottom surface of the pad. This situation is illustrated in FIG. 21( a)where part of the laser beam 34 emanating from the laser interferometer32 is transmitted through the pad 22 to form a transmitted beam 702, andpart of the laser beam 34 is reflected from the backside surface 704 ofthe pad 22 to form a reflected beam 706. The reflected beam 706 createsa considerable direct current (DC) shift in the data signal. FIG. 21( b)illustrates this shift (although exaggerated for purposes of clarity).In this example, the DC shift resulting from the reflected laser lightadds about 8.0 volts to the overall signal. The DC shift createsproblems in analyzing the useful portion of the data signal. Forexample, if the data analysis equipment operates in a range of 0-10volts, amplification of the DC shifted signal to enhance the portion ofinterest is all but impossible without reducing or eliminating the DCcomponent of the signal. If the DC component is not eliminated, theequipment would be saturated by the amplified signal. Reducing oreliminating the DC component electronically requires added signalprocessing electronics and may result in a degradation of the usefulportion of the signal. Even if the DC shift is not as large as describedhere, some signal processing will still likely be required to eliminateit. Accordingly, a non-electronic method of reducing or eliminating thisunwanted DC component is desirable.

It has been found that by creating a diffuse surface 704 on the backsideof the pad 22 in the area constituting the window, as depicted in FIG.21( c), the reflected light from that surface is attenuated. Thus, theunwanted DC component of the data signal is reduced. The diffuse surface704 in effect scatters the non-transmitted laser light 708 rather thanreflecting most of it back towards the interferometer 32. The reflectedsignal from the wafer must also pass through the diffuse surface 704 andin doing so some of it will also be scattered. However, it has beenfound that this does not seriously degrade the performance of theinterferometer.

FIG. 21( d) illustrates the data signal obtained when the diffusesurface 704 is employed. As can be seen, with the elimination of the DCcomponent, the signal can be readily amplified and processed without theneed to electronically eliminate any DC portion.

How the diffuse surface is produced is not of central importance. It canbe produced by sanding the back surface of the polishing pad in thevicinity of the window or by applying a material coating which isdiffuse (e.g. Scotch tape), or in any other way that produces thedesired results.

The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

1. A method of polishing a substrate, comprising: holding the substrateon a polishing pad with a polishing head, wherein the polishing pad issupported by a platen; creating relative motion between the substrateand the polishing pad to polish a side of the substrate; generating alight beam; directing the light beam towards the substrate to cause thelight beam to impinge on the side of the substrate being polished;receiving light reflected from the substrate at a detector to generate atime-varying interference signal; and computing a measure of uniformityfrom the interference signal based on a difference between thetime-varying interference signal and a time-varying base signal.
 2. Themethod of claim 1, wherein creating relative motion includes rotatingthe polishing head.
 3. The method of claim 2, wherein creating relativemotion includes rotating the platen.
 4. The method of claim 3, whereinthe light beam is a laser beam.
 5. The method of claim 1, furthercomprising: comparing the measure of uniformity to a reference; andgenerating an alert when the measure of uniformity diverges from thereference by more than a predetermined amount.
 6. A method of polishinga substrate, comprising: holding the substrate on a polishing pad with apolishing head, wherein the polishing pad is supported by a platen;creating relative motion between the substrate and the polishing pad topolish a side of the substrate; generating a light beam; directing thelight beam towards the substrate to cause the light beam to impinge onthe side of the substrate being polished; receiving light reflected fromthe substrate at a detector to generate an interference signal; anddisplaying a characterizing waveform for an operator to see, thecharacterizing waveform presenting data for a period of time thatextends over multiple cycles of the interference signal.
 7. The methodof claim 6, wherein creating relative motion includes rotating thepolishing head.
 8. The method of claim 7, wherein creating relativemotion includes rotating the platen.
 9. The method of claim 8, wherein alaser interferometer generates the light beam and receives the lightreflected from the substrate.
 10. The method of claim 6, furthercomprising: receiving from an operator input indicating two points onthe waveform; computing a frequency from the two points on the waveform;generating filter coefficients from the frequency; using the filtercoefficients to generate one or both of a low frequency signal or a highfrequency signal from the interference signal; and computing a measureof uniformity from one or both of the low frequency signal or the highfrequency signal.